This disclosure relates generally to thermographic inspection. More particularly, but not by way of limitation, this disclosure relates to high-frequency lock-in thermography using single photon detectors.
Thermography is a nondestructive, nonintrusive, noncontact mapping of thermal patterns or “thermograms,” on the surface of objects through the use of some type of infrared (IR) detector. The principle of lock-in thermography (LIT) consists of introducing periodically modulated energy (e.g., electrical impulses) into an object (e.g., solar cells, integrated circuits, and stacked die geometries) and monitoring the periodic (relative) surface temperature coincident with application of the input energy. One application of LIT technology is the characterization of shunts in solar cells. Shunts are sites of locally increased forward current density. Since a solar cell is forward biased in operation, any shunt current reduces the efficiency of the solar cell. Shunts may be caused by electrical defects of the pn-junction, which may be generated by lattice defects, as well as by technological imperfections of the production process. Another application of LIT technology is the functional testing of electronic devices like integrated circuits. Large differences in the IR emissivity between metalized pathways (i.e., metallizations) and bare silicon layers allows identification and localization of circuit defects. Yet another application of LIT is the localization of gate oxide integrity (GOI) defects in Czochralski-grown silicon metal-oxide semiconductor (MOS) structures. Gate oxide integrity defects are local sites of reduced breakdown voltage. Once defects are identified and localized by LIT, additional microscopic and analytical investigations may be used to clarify the nature of the defects and to find ways to avoid them.